Italy has been described as the world's biggest open-air museum.

And with illegally excavated antiquities, looting of unguarded, centuries-old churches and smuggling of precious artworks, it's also an art theft playground.

But thanks to an elite police squad, Italy is also at the forefront in combating the illicit trade in artworks — believed to be among the world's biggest forms of trafficking and estimated to be worth billions.

Italy's Carabinieri for Protection of Italy's Cultural Heritage recently sponsored an exhibit at Rome's Palazzo Barberini museum, showcasing some of its biggest successes.

A fifth grade class of a Roman elementary school came to see some 200 artworks that were stolen and then recovered.

Lt. Sebastiano Antoci, a 20-year veteran of the elite squad, told the kids how its investigations work.

"We tail suspects or use wiretaps so we can listen to the bad guys' phone calls or we check their bank accounts. And when we're out in the field," he said, "we look like everyone else, we don't wear uniforms."

The fifth-graders listened attentively to the art detective as he pointed to two medieval frescoes.

"We recovered the lamb in Switzerland," he said, "and the Christ in the United States. They're back together again for the first time since they were stolen" — in 1978 from a small church in Guidonia, a town south of Rome.

In 1969, Italy created the world's first specialized police force to combat art crime. It now numbers 280 investigators who also safeguard artworks in regions struck by floods and earthquakes. The unit also combats antiquities trafficking fueled by conflicts in the Middle East and Afghanistan.

At the Rome exhibit, Antoci showed the schoolchildren a marble sculpture that depicts a man and his two sons. It originates from the ancient Syrian city of Palmyra — which recently came under ISIS control.

Adding to his knowledge of art history the excitement of a detective tale, Antoci tells the kids the story of the sculpture, which was tracked down as part of an investigation into financial irregularities and dates back some 2,000 years. "It's a funerary sculpture," he tells them. "The terrorists smuggled it out of Syria and put it on the illicit antiques market. We tracked it down to an Italian businessman in Piedmont, who bought it just it a few months ago."

Gen. Fabrizio Parulli, the commander of this unique police force, explains what's needed to become a good art sleuth. "First of all, you need to be a good investigator," he says.

Speaking in his Rome office — located in a Baroque square that looks like an opera stage set — Parulli says his agents start as police officers and then get specialized training in art history, archaeology, restoration and recognizing counterfeit works.

But the heart of the investigative work is done elsewhere, in a large barracks in Rome's Trastevere neighborhood.

Sitting at a computer, Lt. Francesco Ficarella demonstrates the jewel in the crown of the cultural heritage protection squad — a database known as Leonardo, containing names and photos of close to 6 million registered artworks, mostly from Italy. Of those, 1.2 million are listed as stolen, missing, illegally excavated or smuggled.

Leonardo, he says, "is a crucial instrument not only for our national police forces but also for those abroad — it's the biggest artworks database in the world," he says.

The squad's recovery record is high. In 2014, it managed to recover 137,000 works with an estimated value of $500 million.

Until they're returned to the owners, recovered pieces are warehoused on the ground floor of the Trastevere building. Behind an armored door, tens of thousands of artworks are stored — wooden crucifixes, marble busts, bronze statues and hundreds of paintings, all carefully labeled.

These recovered pieces serve as evidence in criminal cases that are still open.

One of them, "Leda and the Swan," by 16th century painter Lelio Orsi, was auctioned for $1.6 million in New York. Smuggled out of Italy, it was tracked down, thanks to cooperation from U.S. Immigration and Customs Enforcement.

But there's one piece that has eluded this elite art squad for almost three decades: a 6-square-foot canvas of the Nativity by the Baroque master Caravaggio. It was stolen in Sicily in 1969, the same year this special unit was created.

Lt. Calogero Gliozzo says the painting's whereabouts were known until the early 1980s. "We know the names of the robbers and we know the Mafia family that was hiding it," he says, "but then there was a Mafia war and we lost track of the painting."

One Mafia informant told police he had heard that the canvas had been destroyed by rats at a farm where it was hidden.

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